In offshore drilling operations in deep water, the operator will perform drilling operations through a drilling riser string. The drilling riser string extends from a floating platform, such as a drilling ship, to a subsea wellhead or subsea tree assembly on the seafloor. The drilling riser string is made up of a number of individual riser joints or sections that are secured together to form the drilling riser string. The drilling riser string forms a central tube for passing a drill pipe from the floating platform to the wellhead on the sea floor. The drilling riser string normally has a number of auxiliary conduits that extend around the central tube. The auxiliary conduits may serve several purposes, such as supplying hydraulic fluid pressure to the subsea blowout preventer and lower marine riser package.
Typically, the central tube of a drilling riser joint has a pin member on one end and a box member on the other end. The pin end of one riser joint stabs into the box end of the adjoining riser joint. In one type of riser joint, flanges extend outward from the pin and box. The operator connects the flanges together with bolts spaced around the circumference of the coupling. In another type of riser, individual segments or locking segments are spaced around the circumference of the box. A screw is connected to each locking segment. Rotating the screw causes the locking segment to advance into engagement with a profile formed on the end of a pin.
In these systems, a riser spider or support on a riser deploying floor moves between a retracted position into an engaged position to support previously made-up riser joints while the new riser joint is being stabbed into engagement with the string. Wave movement can cause the vessel to be moving upward and downward relative to the riser when the riser is in operation.
In both types of risers, workers use wrenches to make up the bolts or screws. Personnel employed to secure the screws or the bolts are exposed to a risk of injury. Also, the process of making up the individual bolts is time consuming. Often when moving the drilling rig from one location to another, the riser has to be pulled and stored. In very deep water, pulling and rerunning the riser is very expensive.
A technique has been developed that uses a cam ring and dogs to secure drilling riser joints together. Each riser joint has a box end and a pin end. The pin end of one drilling riser joint is disposed within the box end of an adjoining drilling riser joint. The box ends of each drilling riser joint have dogs that are driven into engagement with the pin ends of the adjoining drilling riser joints by moving the cam ring axially. Friction between the dogs and the cam ring maintains the cam ring positioned to drive the dogs against the pin end of the adjoining drilling riser joint. No bolts or screws are used to connect drilling riser joints using this technique.
However, it is conceivable that friction may not be sufficient to maintain the cam rings at their desired axial positions so that the cam rings drive the dogs against the pin ends of the adjoining drilling riser joints. Were a cam ring to move from its desired axial position, its dogs could back out from the pin end of the adjoining drilling riser joint. If that were to occur, the drilling riser joints may disconnect from each other.
Therefore, a more effective technique is needed to secure drilling riser joints together. In particular, a technique is desired that would enable adjoining drilling riserjoints to be connected quickly and remain connected during operation.